AR issues

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Re: AR issues

Postby crbutler on Thu Sep 23, 2021 8:06 pm

The way you wrote that confused me.

So the lower worked with a different upper? If so, it’s not the lower in isolation.

Did the upper work with a different lower? If so, it’s not the upper in isolation.

If you have them, swap in a known working different BCG and bolt. If that solves the problem, then try just the new bolt and just the new BCG. You will isolate the faulty part.

I don’t get the point of 2 different uppers working with other lowers, unless you mean it’s not ammo related?

If the rifle doesn’t load reliably from bolt lock from the magazine, you have a big problem- the tolerances between upper and lower don’t match up. Probably means the upper is pooched and out of spec since the lower worked with a different upper.

If it’s only not loading from firing after the above, you’ve isolated it to the gas system. What to do after you’ve isolated to that is beyond my pay grade…. Gunsmith time in my case. Change port size, different gas system (rifle/carbine), change gas block…
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Re: AR issues

Postby Rip Van Winkle on Fri Sep 24, 2021 7:58 am

Drizzle wrote:I am having AR issues. The rifle (carbine length) will not feed reliably. It does eject, but will not take a round off of the top of the magazine. A few tries already:

I have a new KP15 polymer lower, which recommends Gen 3 AR mags or US GI. I tried brand new 30 and 20 rounders with new ammo. No luck. I lubed the BGC with SLP 2000. I cleaned the chamber with a bore brush previously.

I'd rather not fool with switching buffers. The KP Arms buffer in the lower says it should work fine with carbine length gas systems.

The upper is an SW MP15 with a military profile 14.5 pinned barrel. I have not changed the gas port at all.

I had this same problem previously with the SW lower as well, but did not shoot it as much. That was a demo rifle I got several years ago, so I did not know the history, but it should have run fine. The KP is brand new.

It has in the past liked 20 rounders more than 30s.

I'm thinking either a gas issue or a buffer issue, but I'm open to suggestions. Thanks.

Drizzle wrote:Would rarely lock back on an empty mag after firing one round, be it Gen 2 or 3. Sometimes would not feed from Gen 2 on bolt drop. All ejected fine, in the same general area about 4 o'clock at six feet or so.

BCG and bolt looked ok. Would stand nose up on an extended bolt, so the gas rings are likely ok.

Switched BCGs, made no difference. Switched uppers, made a big difference. Ran reliably with two different lowers on a different upper that I know has a good gas hole and blocks.

I didn't have any but 55gr to check. What I had in the pile.

So, undergassed? Needs a new gas block or bigger port in the barrel? Thinking about a low pro gas block (it's a standard tower now) and/or maybe a new barrel.


I'm quoting both your main posts just to keep my thoughts in order.

You have 2 uppers and 2 lowers.

Upper #1 is a S&W, Upper #2 is a ?.

Lower #1 is a KP15, Lower #2 is a ?

Your failure to feed and lock back issues happen when upper#1 is mated to lower #1. You don't have any issues when upper #2 is mated to lower #1 or when upper #1 is mated to lower #2.

Do both uppers have carbine length barrels and gas systems?

Are both lowers carbine length buffers, spring and tube? If yes, are the buffers the same? do either have any markings? GI carbine buffers come in at least 3 different weights.

If the answer to the above is yes, they're the same, then we're dealing with a gas issue. Either the block is misaligned or the barrel hole is too small.
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Re: AR issues

Postby Drizzle on Fri Sep 24, 2021 3:14 pm

You have 2 uppers and 2 lowers. YES

Upper #1 is a S&W, Upper #2 is a ?. Unk, built on an Aero lower

Lower #1 is a KP15, Lower #2 is a ? Aero

Your failure to feed and lock back issues happen when upper#1 is mated to lower #1. You don't have any issues when upper #2 is mated to lower #1 or when upper #1 is mated to lower #2. First part, Yes. Then, Upper 2 + Lower 1 or 2 = Works fine. Upper 1 + Lower 1 or 2 = no good.

Do both uppers have carbine length barrels and gas systems? One is carbine, one is shorter.

Are both lowers carbine length buffers, spring and tube? If yes, are the buffers the same? do either have any markings? GI carbine buffers come in at least 3 different weights. One is carbine, one is shorter. No markings (I'm a little vague for obvious reasons).


If the answer to the above is yes, they're the same, then we're dealing with a gas issue. Either the block is misaligned or the barrel hole is too small.


To recap, either lower works with the shorter, newer upper. The bolt will not lock back on the last round on the Smith upper and KP lower. I did not rest the other upper and lower for this, as it has worked fine to date.

Thanks for all everyone's suggestions.
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Re: AR issues

Postby Holland&Holland on Fri Sep 24, 2021 8:32 pm

???? One is pistol length?

Is it a registered SBR?
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Re: AR issues

Postby Holland&Holland on Fri Sep 24, 2021 11:04 pm

So it is a pistol that is not cycling correctly?
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Re: AR issues

Postby Rip Van Winkle on Sat Sep 25, 2021 12:03 pm

The buffer is still a question mark in my head, but I'm thinking your upper is undergassed.

IIRC the spec for the gas port is .086" to .096". Unless you have numbered carbide drills and a mill, widening the gas port probably isn't a job you'd want to tackle yourself.

You mentioned your desire to replace the FSB/gasblock with another. If it were me, I'd sell the S&W upper and get a replacement more to your liking.
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Re: AR issues

Postby Drizzle on Sat Sep 25, 2021 3:23 pm

Thanks, Rip. A new upper or a new barrel and gas block are likely in my future.
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